Up to now cellular systems have base stations using omni-radiating antennas or antennas radiating in a sector (typically 120° coverage per sector). The antennas cover the whole cell and no knowledge of the position of the mobiles is used.
To increase the coverage and capacity of future systems, large efforts have been put into development of transmit diversity (TX-div) schemes/systems using multiple antenna units. TX-div utilizes multiple signal source generation to be transmitted via more-or-less independent (non-correlated) propagation channels to the receiver. The aim is to improve the reception of the received total signal quality. This is commonly obtained through coherent combining of the several source signals.
Different TX-div methods/schemes commonly put different hardware (HW) requirements on the radiated signals and their relative behavior. These requirements can usually be associated to the relative time, phase and/or amplitude of the radiated signals.
Another method to create capacity/coverage increase in conventional cellular systems, is to use adaptive antenna arrays at the Radio Base Station (RBS).
The narrow-beams created by beam forming in the adaptive antenna system can be used to increase the coverage by directivity and to reduce the interference in both the uplink and the downlink. Thus, the idea is to avoid distributed energy which no one can make use of, i.e. minimizing the interference in the system. In the cellular systems both broadcast information (i.e. information addressed to all users in the coverage area) and dedicated information (i.e. information to a specific mobile terminal) are transmitted simultaneously from an RBS.
Simultaneous transmission in several beams requires coherent signal paths from the creation of the signal up to the antenna, including feeder cable coherency or by additional signal and antenna hardware. The signal coherency may set a number of requirements on the implemented system hardware, not necessarily the same hardware requirements as for the TX-div.
Increasing the coverage and capacity through TX-div schemes involves multiple signal source generation transmitted via in principle non-correlated propagation channels to the receiver. To achieve this, multiple antenna units are used. The most common configuration to be proposed is a set-up having two identical antennas separated by a distance sufficiently large. The two antennas illuminate a coverage area, which in principle is the same for both antennas, see for instance FIG. 1.
Different methods can be used to support the identification and/or combination of the source signals, e.g. delay diversity, frequency diversity, polarization diversity, different identifier (code) for signal acquisition and combination, feedback correction (i.e. receiver measurements communicated back to the transceiver for correction of transmitted signals).
In all common cellular systems (GSM, Global System for Mobile telecommunications; PDC Pacific Digital Cellular system; TDMA-IS136 Time Division Multiple Access; EDGE Enhanced Data rates for Global Evolution; UMTS Universal Mobile Telecommunications System) the TX-div is proposed/used in the downlink, i.e. multiple source signals from the base station are transmitted towards the mobile receiver.
For example in the Universal Mobile Telecommunications System Frequency Division Duplex (UMTS-FDD) (WCDMA, Wideband Code Division Multiple Access) there are several TX-div modes defined in the standard [1] as the open-loop Space Time Transmit Diversity (STTD), closed-loop model and mode2. The mentioned WCDMA TX-div modes above are regarding configurations and schemes utilizing two TX-div branches. In the future it may be standardized even schemes for higher order number of TX-div branches.
In these schemes, there are rather strict requirements on the relative phase, amplitude and/or time accuracy between the signal paths in the TX-div transmission branches
To utilize the potential performance (capacity/coverage) advantages of multi-beam, especially for two fixed-beams as a TX-div solution, requires simultaneous transmission for both cell and narrow beam coverage. A number of issues that must be fulfilled for a cost-effective system have been identified:
Less or equal amount of hardware resources should be required as for a conventional TX-div system with two sector coverage antennas. Specifically, for UMTS-FDD (WCDMA), there shall be no change of RBS (Node B) hardware configuration for TX-div, except antenna hardware and mounting. Requirements on components and/or subsystems must not be increased.
In order not to introduce additional complexity it is required that no additional coherency requirements are introduced in the system. In general, due to vector addition of transmitted signals, simultaneous transmission in two beams requires coherent signal paths from the creation of signals up to the antenna, including feeder cable coherency. Otherwise the radiation pattern will be uncontrolled and can have significant variations, including possible directions with nulls in the radiation pattern. Such coherent signal paths are very delicate to achieve in an installed product with several years of expected lifetime. That kind of solution will include calibration loops and control functions that are expensive to introduce in the system. The issue is to create the coherent antenna system behavior without requiring signal coherency.
Capacity/Coverage: The solution must not limit the possibility to utilize the full potential of the Two Fixed-Beam TX Diversity system. The potential of such a system is expected to achieve better performance than a conventional TX-div system.
No standardization changes are desired: One major requirement is that no interaction with the mobile/terminal is allowed outside the standard protocol. The solution should be transparent to the system.